With reference to FIG. 1, which is a flow chart illustrating rekeying steps of a known rekeyable lock cylinder, the main steps during rekeying process are to add a thrust to the first matched key for making the sliding block move and driving the first rack components to release engagement with the second rack components and to turn the second matched key (new key) for making the sliding block restore and driving the first rack components to reengage with the second rack components. However, general users often doesn't insert the second matched key (new key) into right position to turn the plug during the step of turning the second matched key, so that the new key cannot work to release locking state. Hence, it is extremely serious topic to design a structure of rekeyable lock cylinder for ensuring rekeying operation is not allowed if key is not completely inserted.